Abstract

The metabolic fate of two commonly occurring dietary terpenes in Australianmarsupial eucalypt folivores has been studied. The detoxification mechanismsemployed by these folivores were considered from an ecological perspective.Interactions between herbivores and their respective diets has received considerableattention over the last quarter of a century. A popular hypothesis by Freeland andJanzen (1974) states that feeding behaviour of herbivores is influenced by limitationsof the body's mechanisms for detoxifying plant secondary metabolites (PSM).However, very little is understood about the physiological processes and limitationsof detoxification in wild animals. This study provides an insight into these processesin marsupial eucalypt folivores from a range of feeding niches.An interspecies comparison was made of the metabolism of a bolus dose of themonoterpene p-cymene in a generalist herbivore, the brushtail possum (Trichosurusvulpecula), and three specialist folivores, the greater glider (Petauroides volans), theringtail possum (Pseudocheirus peregrinus) and the koala (Phascolarctos cinereus),as well as in the laboratory rat (Rattus norvegicus) which provided a directcomparison to published data. Each animal was dosed, intragastrically, with singledoses of p-cymene. Urine and faeces were collected pre- and post-dose.Chronic ingestion of 1,8-cineole, emulating its natural occurrence in the diet ofbrushtail possums and koalas, was also studied. Possums were fed an artificial diet inwhich 1,8-cineole concentrations were manipulated to cover a range ofconcentrations while koalas were fed Eucalyptus cephalocarpa, which has a terpeneprofile dominated by 1,8-cineole.Metabolites were identified by extraction, gas chromatography and massspectrometry. Standards for many metabolites were isolated from urine to allowquantitation. A novel p-cymene metabolite, 2-p-carboxyphenylpropan-1,2-diol, wasidentified in the koala. Twelve.novel 1,8-cineole metabolites were identified frombrushtail possum and koala urine. Eleven of these were isomers of hydroxy cineolicacid. The isomeric structure and partial stereochemistry for 7-hydroxy-9-cineolicacid were determined by NMR and mass spectrometry.Observational data from chronic ingestion of 1,8-cineole in possums demonstrated athreshold in maximum daily intake in most possums between 3 and 4 g/kg. Thepattern of metabolites excreted in the brushtail possum during chronic ingestiondemonstrated induction of enzymes responsible for producing the more oxidisedmetabolites during the first days of ingestion. There was no evidence of saturation ofmetabolic pathways associated with the larger intakes of 1,8-cineole. For both terpenes, species-specific patterns of metabolite excretion were evident andreflected the natural occurrence of eucalypt leaves in the different diets. The numberand degree of oxidation of metabolites, as well as the role of conjugation, varieddepending on the natural feeding behaviour of the animals.To facilitate studying the pattern of metabolism, terpene metabolites were groupedaccording to the total number of oxygen atoms (up to four) acquired during oxidation. Overall there was a progressive increase in the proportion of more extensivelyoxidised metabolites; from the rat, to the generalist and through to the specialistfolivOres. The generalist utilised a multiplicity of non-specific oxidative pathwaysproducing an array of metabolites covering all degrees of oxidation (9 metabolites ofp-cymene and 18 metabolites of 1,8-cineole). On the other hand, the specialists hadhigh capacity and specific oxidative pathways resulting in relatively few metabolites,all of which were radically oxidised.Glucuronidation was important in the generalists, compensating for their inability toexcrete the same degree of radically oxidised metabolites as specialists.Approximately 40 - 50 % of p-cymene metabolites and up to 60 % of 1,8-cineolemetabolites were conjugated with glucuronic acid in brushtail possums. Nosignificant conjugation of metabolites was observed in the specialists.Increased polarity, whether achieved by glucuronidation or extensive oxidation,presumably results in the same overall enhanced capacity to excrete metabolites. It isproposed that, for specialists, oxidative efficiency reduces the necessity forsubsequent conjugation, conserving glucuronic acid, a valuable resource in anutritionally limited diet. The brushtail possum, however, consumes a varied andarguably better quality diet and can afford to excrete glucuronic acid in thedetoxification of terpenes. Given that PSMs, such as terpenes, are always present inthe eucalypt leaf diet of these folivores, these different strategies indicate thatadaptation of detoxification mechanisms vary with dietary specialisation.

Item Type:

Thesis
(PhD)

Keywords:

Plant metabolites, Herbivora, Herbivora, Terpenes

Copyright Holders:

The Author

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